High pressure reactors for the polymerization of ethylene typically operate at pressures in excess of 1500 bar, and sometimes as high as 3000 bar. Moreover, the economic success of the process depends on the plant having a long operating lifetime and on keeping downtime to a minimum. High pressure ethylene polymerization reactor systems typically employ both a primary compressor that compresses the ethylene feedstock up to a pressure of, for example, 300 bar and a secondary compressor that further compresses the ethylene from the outlet pressure of the primary compressor up to the reactor pressure. Such a secondary compressor is mechanically complex and is subject to enormous mechanical forces, but is nonetheless required to operate at a high throughput reliably and safely over a lifetime of several decades in order for the process to be economically viable. Accordingly, the successful design and operation of the secondary compressor is critical to the commercial viability of the polymerization process.
The economic viability of the production of polyethylene and polyethylene copolymers in high pressure reactors is also heavily dependent on the scale of the process, that is, the tonnage of product produced per year. However, the mechanical demands on the secondary compressor, particularly the loads applied through the drive train, increase as the required throughput increases. Therefore, there is a need for secondary compressors which can operate reliably and safely at high throughputs. Note that for the purposes of this application, we will refer to the compressor as the secondary compressor, however, an initial reciprocating pump (instead of a primary compressor) may be used to bring the ethylene to a pressure of, for example, 300 bar, or, alternatively, the compressor frame could be modified to allow a single compressor to compress the ethylene from 1 bar to the operating pressure.
Secondary compressors for use with high pressure polymerization reactors are typically two-stage reciprocating compressors having, say, six or eight cylinders arranged in a compressor frame and having a common crankshaft driven by an electric motor standing at one end of the compressor frame. It is usually necessary to mount the compressor on foundations specially adapted to minimize vibration. One approach to developing secondary compressors of higher throughput has been to increase the size of the cylinders. However, that approach suffers from the disadvantage of also increasing the loads applied to the components of the cylinders and frame running gear components, particularly the connecting rods, bearings and those components that resist the greater pressure end load resulting from larger plunger diameters. A second approach has been to increase the number of cylinders in the compressor frame. However, that approach requires an increase in the length of the crankshaft and an increase in the power transmitted through the crankshaft and those factors limit the number of cylinders that can be included in a compressor frame.
A third approach to increasing the compressor throughput has been to include a second compressor frame on the opposite side of the motor. However, due to the difficulty inherent in attempting to perfectly align the crankshafts of two separate compressor frames, it has proved necessary to couple at least one of the compressor frames to the motor via a flexible coupling to avoid unsustainable stresses on the crankshafts, motor, and associated components due to imperfect alignment. One known type of flexible coupling used with secondary compressors includes packs of thin disc membranes, through which the torque applied by the motor is transmitted to the crankshaft of the compressor frame, and which can flex as they rotate, thereby accommodating the strains resulting from a slight misalignment of the compressor frame crankshaft and the motor driveshaft. However that type of flexible coupling has been found, in use, to suffer from problems which can cause the membranes to break up and fail. Failure of a flexible coupling may be hazardous to people at the plant and may cause a sudden plant shut-down requiring a lengthy clean up and re-commissioning period. Furthermore, the magnitude of the torque that can be transmitted by this type of coupling is limited by the size of the thin disk membranes that can be manufactured. Accordingly, there remains a need for a secondary compressor that can operate at high gas throughput reliably over a long working lifetime.